In recent years, optical wireless transmission technology has attracted attention as means for transmitting information signals with light. The optical wireless transmission technology includes, for example, an optical wireless LAN to communicate information among a plurality of computers and a technique to transmit video and audio information to audio-visual devices. The optical wireless transmission transmits and receives optical beams modulated by digital information among devices each having a receiver-emitter.
To correctly communicate information with the optical wireless transmission, optical axes must be aligned between the receiver-emitters. Namely, each receiver-emitter must have an optical axis adjusting function.
If a light beam emitted from a light emitting element on a transmission side has no directivity, optical axis adjustment must be carried out by a photodetector on a reception side. If the light emitting element on the transmission side has directivity, optical axis adjustment must be carried out on each of the transmission and reception sides. To carry out high-speed communication in a gigahertz band, securing a transmission S/N ratio is important. For this, a light beam having a small diameter is employed and precise optical axis adjustment is needed.
An optical axis adjusting mechanism to realize a precision optical axis adjusting function is disclosed in Japanese Patent Application Laid-Open Publication No. 2003-8515. According to this disclosure, a two-axis driving mechanism supports a light receiver-emitter and deflects a light receiving/emitting direction of the light receiver-emitter in a direction defined by two axes.
There is a light detecting apparatus having an angle controlling mechanism. To reduce the size of a communication system and adjust an optical axis at high speed, this apparatus employs a mirror that is deflected along two axes. With this mirror, the apparatus makes a light beam incident to a photodetector on a reception side. In this apparatus, the mirror is deflected in a direction defined by two axes to search for an orientation of a light emitting element installed on a transmission side and align optical axes between the light emitting element and the photodetector.
To carry out the search operation of finding an orientation of the light emitting element installed on the transmission side, the light detecting apparatus monitors a light intensity detected by the photodetector on the reception side, deflects the mirror according to preset conditions, stores an orientation of the mirror when the detected light intensity reaches a maximum, and adjusts the mirror in the stored orientation after completing the mirror deflecting operation based on the preset conditions.
To more precisely control optical axes with this light detecting apparatus, a light beam from the light emitting element on the transmission side is made incident to a two-dimensional position detector through the mirror that is deflectable along two axes. According to an incident position of the light beam on the two-dimensional position detector, the light detecting apparatus feedback-controls a deflecting direction of the mirror. This is closed loop control that can precisely adjust optical axes between the light emitting element and the photodetector.
The light detecting apparatus mentioned above employs an angle controlling mechanism including the deflection mirror and two-dimensional position detector. The closed loop control of the apparatus is achievable only when a light beam is made incident to the two-dimensional position detector. If the light beam is out of the two-dimensional position detector due to a large optical axis deviation, the apparatus is unable to achieve the closed loop control. Accordingly, this type of light detecting apparatus needs, if an optical axis deviation is large, to carry out a search operation until an optical beam is made incident to the two-dimensional position detector, before conducting a servo lock operation.
The search operation by the light detecting apparatus is required to surely and quickly find an orientation of a light emitting element installed on a transmission side. For this, the search operation must be carried out for a wide range of angles at high speed or high frequency. In addition, the light detecting apparatus must be capable of deflecting the deflection mirror for a wide range of angles at high speed or high frequency.
The speed (frequency) of deflecting the mirror is restricted by the mechanical characteristics of the mirror. When the mirror is deflected at high speed (high frequency), the mirror may abnormally vibrate due to mechanical resonance, to deteriorate the search operation. This problem becomes more obvious when the mirror is deflected more widely.
Due to the problem, the light detecting apparatus of the related art is unable to sufficiently increase a mirror deflecting speed (frequency), or shorten a search time, or decrease an optical axis adjusting time. When the diameter of a light beam emitted from the light emitting element on the transmission side is small, a high-density search operation is needed, and the related art needs a very long time to carry out such a high-density search operation.